const

The const keyword (part 2)

Posted by Strainu on May 29, 2007
C, C++ / No Comments

In part 1 we introduced the const keyword. Today we’ll talk about constant pointers.

Let’s say you want to define a constant pointer. Which of the following declarations will you use?

const int* v1;
int* const v2;
const int * const v3;

The answer is: the second or the third. The first declaration defines a pointer to a constant integer and the third one defines a constant pointer to a constant integer.

Another interesting case is when defining a constant char array (credits go to Ulrich Drepper, link via RazvanD):

int main(void)
{
const char s[] = "hello";
strcpy (s, "bye");
puts (s);
return 0;
}

Although this code will give a warning (passing `const char *’ as argument 1 of `strcpy(char *, const char *)’ discards qualifiers is the exact message on Dev-C++), it will run, because s is allocated in the heap, so it is treated much like a pointer. You can force the value to be constant by adding the static keyword, wich will force the compiler to allocate s in read-only memory:

static const char s[] = "hello";
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The const keyword (part 1)

Posted by Strainu on May 24, 2007
C, C++ / 1 Comment

This article will not actually present any tricks, it will be an introduction in the const keyword. In part 2, we will present the const and volatile pointers, which behave a little weird.

First of all, let’s see what the const modifier means in the C standard. Basically, a const variable is one who’s value can’t be changed. Actually, things are not so simple – as we’ll see later, you can change a constant variable. The standard states that “If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined.” On some architectures, constant variables are put in a special section (sometimes called .rodata – from Read Only DATA) of the program by the compiler.

If you want to define a constant in C/C++, you can write:

const int v1 = 0; //the usual way
int const v2 = 1; //also legal

Both declarations mean the same thing: define a new integer with a fixed value. So, if you want to change the value of v1, how would you do it? By using pointers:

const int v1 = 0; //define a constant
int* v2 = &v1; //define a pointer to v1
*v2 = 5; //change the value

Of course, you shouldn’t do that, as the are no guarantees that the result will be what you expect it to be, but with most compilers, v1 will be 5 after running the code presented above.

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